The present invention generally relates to the field of optical communications and more particularly to methods and apparatus for determining nonlinear properties of optical devices and fibers.
Nonlinear effects that cause unwanted impairments may occur during the propagation of light pulses in fibers and other optical devices. For example, self phase modulation (SPM) and cross-phase modulation (XPM) are known to be limiting factors in long-haul optical networks. SPM and XPM effects correspond to an induced temporal phase that is related to the temporal intensity of the pulses themselves (in the case of SPM) or to the temporal intensity of another optical source or a combination of optical sources (in the case of XPM). The coupling between the intensity and phase depends upon the medium, the characteristics of the interacting waves (for example the state of polarization and wavelength) and the nonlinear interaction. For example, in an optical fiber, SPM manifests itself on a signal of intensity I(t) as an intensity-dependent phase φ(t) such as φ(t)=ΓI(t).
Nonlinear effects are also beneficially used in a wide variety of applications, such as wavelength conversion using a semiconductor optical amplifier, or pulse compression in a nonlinear fiber. Signal processing and pulse compression applications can require a high nonlinear index, since this decreases the required input peak power to achieve a given nonlinear phase shift.
It can be appreciated by those skilled in the art that measurement of the linear and nonlinear properties of optical devices and fibers is an important task in optical telecommunications since these properties can directly impact the propagation of light through devices and fibers.
Various techniques to measure nonlinear effects have been proposed. One proposed technique is based on the generation of an optical signal having known properties, and propagation of the optical signal through a device under test at a power level sufficient to induce modification of the signal via the particular nonlinear effects to be measured or characterized. Using such a technique, the temporal electric field of a short optical pulse can be measured before and after propagation through the device under test, and the comparison of the temporal phase before and after propagation can be used to determine a measurement of the nonlinear coefficient of the device. A limitation of this technique is that the generation of short optical pulses can be complex and costly. Furthermore, pulse characterization is usually devoted to pulses shorter than 100 ps, and these pulses can be modified by chromatic dispersion and polarization-mode dispersion during propagation in the device under test, thereby complicating any analysis of the results.
Another known technique involves propagating light from two CW lasers through a device under test and measuring the optical spectrum after propagation to obtain a nonlinear coefficient. A limiting factor of this technique is that since the wavelengths of the two lasers must be very close to avoid the effects of chromatic dispersion and polarization-mode dispersion, the use of a costly high-resolution optical spectrum analyzer is typically required.
Finally, techniques for measuring the nonlinear refractive index have been demonstrated based on self-phase modulation, cross-phase modulation, four-wave mixing or modulation instability. These techniques typically do not directly measure the phase shift, but instead make a determination from a measured experimental trace, for example, the power and frequency of sidebands in the optical spectrum. Since the determination of the nonlinear properties of the medium is indirect, it can be subject to errors that are difficult to track.